1. Field of Invention
The present invention relates to a thermal head and a manufacturing method therefor, and a thermal printer, the thermal head being used in the thermal printer often mounted to a portable information equipment terminal typified by a compact hand-held terminal and being used to perform printing on a thermal recording medium based on printing data with the aid of selective driving of a plurality of heating elements.
2. Description of the Related Art
Recently, the thermal printers have been widely used in the portable information equipment terminals. The portable information equipment terminals are driven by a battery, which leads to strong demands for electric power saving of the thermal printers. Accordingly, there have been growing demands for thermal heads having high heat generating efficiency.
As a thermal head having high heat generating efficiency, one which has a structure disclosed, for example, in Japanese Patent Application Laid-Open No. 2007-83532 is known.
However, in the thermal head disclosed in FIG. 3 of Japanese Patent Application Laid-Open No. 2007-83532, a substrate (supporting substrate) and a heat storage layer are bonded to each other by anode bonding. Therefore, if a monocrystal silicon substrate having the thermal expansion coefficient of 3.3×10−6 per degree centigrade is used as the substrate, and if soda glass that is inexpensive and has good workability but has the thermal expansion coefficient of 8.6×10−6 per degree centigrade is used for the heat storage layer, a thermal expansion difference occurs between the substrate and the heat storage layer because of the temperature of a heating resistor that rises up to approximately 200 to 300 degrees centigrade when the thermal head is energized. As a result, a warpage or a distortion may occur in the thermal head so that the thermal head cannot contact correctly with thermal recording paper, which may cause a deterioration in print quality.
In contrast, if a monocrystal silicon substrate having the thermal expansion coefficient of 3.3×10−6 per degree centigrade is used as the substrate, and if Pyrex glass that is expensive and has bad workability but has the thermal expansion coefficient of 3.2×10−6 per degree centigrade is used for the heat storage layer, a warpage or a distortion does not occur in the thermal head. However, there are problems that manufacturing cost increases and that manufacturing steps are complicated.
On the other hand, an amount of heat flowing in the substrate side is restricted merely by using a thin glass plate having low thermal conductivity as the heat storage layer. However, in order to secure mechanical strength of the heat storage layer, it is necessary to set the thickness of the heat storage layer to be larger than 20 μm. As a result, there is also a problem that sufficient heat cannot be stored in the heat storage layer when the printing is started, and hence print density becomes low.